Soil Alkalinity
In arid and semi-arid regions, evaporation exceeds precipitation, leaving basic cations (Na⁺, Ca²⁺, Mg²⁺) concentrated at the surface.
Alkaline Soil Formation
- Weathering of minerals releases soluble salts.
- Carbonates (e.g., Na₂CO₃) raise soil pH to 9-10: Na2CO3+H2O→2Na++HCO3−+OH−Na_2CO_3 + H_2O —-à 2Na^+ + HCO_3^- + OH^-Na2CO3+H2O→2Na++HCO3−+OH−
Properties of Acidic Soils
1️. Strongly Acidic Soils (pH < 5.0)
- Al³⁺ is tightly bound to organic matter or present as aluminum hydroxyl cations.
- Hydrolysis of Al³⁺ releases H⁺, lowering pH: Al3++H2O→Al(OH)2++H+text{Al}^{3+} + H2O rightarrow text{Al(OH)}^{2+} + H^+Al3++H2O→Al(OH)2++H+
- Hydrogen & aluminum ions increase soil acidity.
2️. Moderately Acidic Soils (pH 5.0 – 6.5)
- Base saturation is higher than in strongly acidic soils.
- Aluminum exists as hydroxy ions, contributing to pH buffering: Al(OH)2++H2O→Al(OH)3+H+{Al(OH)}_2^+ + H_2O rightarrow text{Al(OH)}_3 + H^+Al(OH)2++H2O→Al(OH)3+H+
3️. Neutral to Slightly Acidic Soils (pH ~7.0)
- Dominated by base cations (Ca²⁺, Mg²⁺, K⁺) instead of H⁺ & Al³⁺.
- Al³⁺ is converted into insoluble gibbsite: Al3++OH−→Al(OH)3text{Al}^{3+} + OH^- rightarrow text{Al(OH)}_3Al3++OH−→Al(OH)3
4️. Acid Sulfate Soils (pH < 4.0)
- Acidity is due to sulfuric acid (H₂SO₄), ferric sulfate, and aluminum sulfate.
- Oxidation of sulfur compounds leads to extreme acidity.